In the petroleum industry, acoustic vibrations along structures such as drill strings, pipelines, and the like have been found to be a useful carrier of information. As will be further noted hereinbelow, both in the drilling of wells and in the transmission of fluids through a pipeline, interpretation of the vibrations which are generated by a drill or by leaks can provide important information concerning the environment and operation of the structure. In addition, active generation of acoustic vibrations can be used for transmission of information along such structures at reasonable data rates. The instant invention is directed to improving the ability to interpret acoustic vibrations in order to retrieve the desired information.
One application in which the interpretation of acoustic vibrations can provide important information is during the drilling of both land and offshore wells, in the exploration for and production of petroleum products. Over the years, the more readily found and accessible petroleum reservoirs have of course been discovered and depleted first. As a result, the exploration and production operations must necessarily concentrate to a greater degree on less accessible and less readily discoverable reserves. In order to reach these locations, the depths of drilling have increased, the locations at which drilling takes place have become increasingly difficult and less accessible, and the drilling operations have necessarily become more complex. Accordingly, drilling operations in the search for and production of petroleum products have become more expensive, with this trend likely to continue in the future. Because of this increasing cost, the accuracy and efficiency of the drilling operation is becoming even more important.
The success and efficiency of the drilling operation depends to a large degree on the quantity and quality of information that the drilling operator has about the sub-surface structure into which the drilling is taking place, and also about parameters concerning the operation of the drill bit as it proceeds into the earth. Many techniques for acquisition and communication of such information have been tried and used in the industry. Recent work has been done, as will be discussed hereinbelow, in acquiring information from the acoustic vibrations of the drill string itself during drilling. In such an application, the drill string serves not only to power and guide the drilling, but also as a communication medium for such acoustic signals. These signals are inherently generated during the drilling operation and communicated via the drill string to detectors. Analysis of the signals provides information about the drilling parameters and the drilling operation itself, and also about the geology into which the drilling is taking place.
An example of a system and method using acoustic vibrations transmitted along the drill string itself to communicate various drilling parameters is described in U.S. Pat. No. 4,715,451, issued Dec. 29, 1987, assigned to Atlantic Richfield Company, said U.S. Patent incorporated herein by this reference. This system measures the motion of, and the strains on, a drillstem in various directions, by way of monitoring such indications as axial, torsional and lateral vibrations, and deflections of the drillstring. The strain generated on the drill string during drilling is indicative of such factors as the impact and rotation of the drill bit, its interaction with the formation into which the drilling is taking place, and the interaction with portions of the drill string above the bit with the surrounding formation. In this system, measurements are made by way of detectors, such as accelerometers and strain gages, which are located in a sub near the top of the drill string and which generate electrical signals corresponding to the vibration and motion detected thereby. Analysis of the electrical signals provides real-time information on parameters such as drillstem vibration and deflection, the location of interaction between the casing and the drillstem, the speed of and load on the drill bit, and other drill bit operating characteristics. Such real-time operating information is quite useful in efficiently and accurately performing the drilling operation.
As disclosed in said U.S. Pat. No. 4,715,451 at column 5, lines 59 through 68, in Rector III, et al., "Extending VSP to 3-D and MWD: Using the drill bit as downhole seismic source", Oil and Gas Journal, (Jun. 19, 1989), pp. 55-58, and in Rector, Marion and Widrow, "Use of Drill-Bit Energy as a Downhole Seismic Source", 58the International Meeting of SEG, paper DEV 2.7, pp. 161-164, analysis of the vibrations communicated along th drill string during drilling is also useful in the seismic prospecting area, where the vibrations generated by the drill bit into the earth are the seismic source signals. In the TOMEX.RTM. (Trademark of Western Atlas International Inc.) system disclosed by Rector III et al., seismic detectors such as geophones or hydrophones detect the reflections of these vibrations near the surface at a location distant from the drilling operation. Detection of the vibrations at the wellhead, as communicated by the drill string, can provide a signature of the source vibrations. Conventional cross-correlation of the vibrations detected by the geophones or hydrophones with the source vibrations communicated through the drill string provides data concerning the location of sub-surface strata and interfaces.
Another system which utilizes the drill string as a medium for the transmission of data is referred to as stress wave telemetry. Stress wave telemetry systems are disclosed in copending U.S. patent applications Ser. No. 188,231 filed Apr. 21, 1988, now U.S. Pat. No. 4,992,997, issued Feb. 12, 1991, Ser. No. 554,030 filed Jul. 16, 1990, and in Ser. No. 554,022 Jul. 16, 1990, all applications assigned to Atlantic Richfield Company, and incorporated herein by this reference. These systems include transmitters, such as solenoids, eccentric motors, and piezoelectric transducers, which intentionally vibrate the drill string in a manner corresponding to the desired data. This data may include information concerning drilling parameters, such as in the above-referenced U.S. Pat. No. 4,715,451. In the case of stress wave telemetry, however, the information is not extrapolated from analyzing the naturally occurring vibrations, but vibrations are generated which are in addition to the naturally occurring vibrations, these generated vibrations corresponding to the drilling parameter and other information transmitted along the drill string.
Another important application utilizing the transmission of acoustic signals along a pipe structure is leakage detection. An example of a system and method using acoustic signals for leakage detection in a pipe is described in my copending application Ser. No. 391,919, filed Aug. 10, 1989 and assigned to Atlantic Richfield Company. As described therein, the leakage of fluids from within the pipe through a crack or faulty joint in the pipe generates acoustic vibrations in the pipe. The system and method described in said copending application Ser. No. 391,919 identifies the existence and location of such leaks by analysis of these acoustic vibrations, for example by measuring selected combinations of axial and torsional vibrations and fluid pressure fluctuations at a point along the pipe.
It has been discovered, however, that vibrations, whether from the drill bit, from stress wave telemetry transmitters, or from leaks, are not communicated through the drill string in an ideal manner, due to the non-ideal response of the drill string to such vibrations. As described in Drumheller, "Acoustical Properties of Drillstrings", J. Acoustic Society of America, 85(3) (March, 1989), pp. 1048-1064, conventional drill strings, which consist of a number of lengths of drill pipe joined by pipe joints, inherently have frequency domain stopbands or deadbands which attenuate acoustical signals at the stopband frequencies. This frequency-dependent attenuation can severely distort some signals. While simple deconvolution of the reflective effects of the ends of the drill string and the bottomhole assembly has been done, such deconvolution has accounted only for effects dependent upon the total length of the drill string and the construction of the bottomhole assemble, and has not accounted for the frequency dependent transmission of the drill string due to such factors as the tool joints between sections of the drill string.
It is therefore an object of this invention to provide a method of reducing the effects of the frequency response on information communicated along a pipe structure such as a drill string, production tubing, or pipeline.
It is a further object of this invention to provide such a method which includes deconvolution of the pipe string response in accomplishing the noise reduction.
It is a further object of this invention to provide such a method where the deconvolution takes into account the passbands and deadbands of a jointed pipe structure.
It is a further object of this invention to provide such a method which stores time delay values of the signals.
It is a further object of this invention to provide such a method where the Fourier transform of the impulse response may be performed and stored, using a small number of points, with no loss of information.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to this specification, together with the drawings.